Method for producing hexanediol-1,6

Abstract

A process for preparing 1,6-hexanediol from a carboxylic acid mixture obtained as by-product in the oxidation of cyclohexane to cyclohexanone/cyclohexanol with oxygen or oxygen-comprising gases and by aqueous extraction of the reaction mixture and comprising adipic acid, 6-hydroxycaproic acid and small amounts of 1,4-cyclohexanediols by esterification of the acids and hydrogenation, by a) reacting the mono- and dicarboxylic acids in the aqueous dicarboxylic acid mixture with a low molecular weight alcohol to form the corresponding carboxylic esters, b) conducting a first distillation to remove excess alcohol and low boilers from the esterification mixture obtained, c) conducting a second distillation to separate the bottom product into an ester fraction which is essentially free from 1,4-cyclohexanediols and a fraction which includes at least the larger proportion of the 1,4-cyclohexanediols, d) subjecting the ester fraction essentially free from 1,4-cyclohexanediols to a catalytic hydrogenation, and e) subjecting the hydrogenation effluent to a distillation to recover 1,6-hexanediol in a conventional manner, comprises using an aqueous dicarboxylic acid mixture comprising more than 20 ppm of cobalt and more than 40 ppm of phosphorus in the form of phosphate and passing this aqueous dicarboxylic acid mixture through a cation exchanger and after the esterification of step (a) through an anion exchanger.

We claim:  
     
       1. A process for preparing 1,6-hexanediol from a carboxylic acid mixture obtained as by-product in the oxidation of cyclohexane to cyclohexanone/cyclohexanol with oxygen or oxygen-comprising gases and by aqueous extraction of the reaction mixture and comprising adipic acid, 6-hydroxycaproic acid and small amounts of 1,4-cyclohexanediols by esterification of the acids and hydrogenation, by 
       a) reacting the mono- and dicarboxylic acids in the aqueous dicarboxylic acid mixture with a low molecular weight alcohol to form the corresponding carboxylic esters,  
       b) conducting a first distillation to remove excess alcohol and low boilers from the esterification mixture obtained,  
       c) conducting a second distillation to separate the bottom product into an ester fraction which is essentially free from 1,4-cyclohexanediols and a fraction which includes at least the larger proportion of the 1,4-cyclohexanediols,  
       d) subjecting the ester fraction essentially free from 1,4-cyclohexanediols to a catalytic hydrogenation, and  
       e) subjecting the hydrogenation effluent to a distillation to recover 1,6-hexanediol in a conventional manner,  
       which comprises using an aqueous dicarboxylic acid mixture comprising more than 20 ppm of cobalt and more than 40 ppm of phosphorus in the form of phosphate and passing this aqueous dicarboxylic acid mixture through a cation exchanger and after the esterification of step (a) through an anion exchanger.  
     
     
       2. A process as claimed in claim  1 , wherein the pretreatment is carried out with fixed bed ion exchangers. 
     
     
       3. A process as claimed in claim  1 , wherein the pretreatment is carried out with two alternately operated fixed beds, which can also be connected in series, until the cobalt content of the outflow from the first bed is more than 90% of the inflow concentration, at which point the first bed is switched off and regenerated to assume the function of the downstream bed. 
     
     
       4. A process as claimed in claim  1 , wherein the pretreatment is carried out with 2 consecutive anion exchanger beds until the phosphate content of the outflow from the first bed is more than 50% of the inflow concentration, at which point the first bed is switched off and regenerated to assume the function of the downstream bed. 
     
     
       5. A process as claimed in claim  1 , wherein the cation exchanger prior to regeneration is washed product-free with water and the wash liquor is mixed into the starting dicarboxylic acid solution. 
     
     
       6. A process as claimed in claim  1 , wherein the anion exchanger prior to regeneration is washed with methanol and the wash liquor is returned into the esterification.